The present invention relates to aperture synthesis radiometers or interferometric radiometers and in particular to a switching scheme for the control of the operation of such radiometers.
Aperture synthesis radiometers are measuring apparatus arranged to synthesise a large aperture using small antenna elements receiving the measuring wave signals and cross-correlating the signals of all pairs. Each antenna element is connected to a receiver arranged to receive the components of the horizontal and vertical electrical field of the antenna element. Normally the antenna of an aperture synthesis radiometer is not mechanically scanned and hence, since its field of view is very wide, the horizontal and vertical polarizations at a given point of the ground do not correspond with the horizontal and vertical polarization of the antenna axes. The result is that the h and v electric field components of the antenna contain each a combination of the H and V field components on the ground and therefore the cross-correlation between the antenna elements of the h or v fields provides the Fourier Transform of a combination of the brightness temperatures which define the Stokes vector, that is, the brightness temperature in linear polarization along H, V and xc2x145xc2x0 from H, and in circular left- and right-hand polarizations.
It is generally assumed that the contributions of the wave components at xc2x145xc2x0 from H (third Stokes parameter) as well as the circular left- and right-hand polarizations (fourth Stokes parameter) are negligible and that the brightness temperature at H and V polarizations can be extracted by linear inversion from the h and v sets of cross-correlations of the antenna fields.
However, during the last few years polarimetric radiometers have shown that the third and fourth Stokes parameters can in fact provide very useful information about the target. As an example, fully polarimetric radiometers are being used to retrieve wind over ocean based on the wind-driven correlation between the H and V electric field components. Another example is the MIRAS radiometer where the third and the fourth Stokes parameters have the potential to provide corrections due to wind for sea surface salinity or for Faraday rotation compensation over land.
If the fourth Stokes parameters are to be retrieved, then four measurements are needed. These are the cross-correlations between antenna elements i and j of the h and v electric fields in all the four possible combinations, that is, (hi hj), (hi vj), (vi hj) and (vi vj). When each antenna element receiver of the interferometer is provided with two parallel channels (one for horizontal polarization and one for vertical polarization) then it is simply a matter of having as many correlators as needed and getting the four sets of correlations desired. But this is not the usual situation one encounters in practice. Interferometric radiometers have a large number of elements and the receivers usually have only one channel in order to save mass and power consumption. Single channel receivers select sequentially the horizontal and vertical polarizations ports of the antenna by means of a switch and therefore it is only possible to perform one of the four correlations at any time, regardless of the number of correlators. The four correlations needed can be obtained by properly selecting the horizontal and vertical ports of each antenna element through an adequate switching sequence. The obvious disadvantage of single channel receivers with sequential polarization switching as compared to dual channel receivers is a reduction of the integration time by a factor 4 at least and this degrades the radiometer sensitivity of the measurements by a factor 2 or more.
The object of the present invention is to provide a novel switching scheme for the signal receivers of an aperture synthesis radiometer with single channel signal receivers, so as to enable fully polarimetric operation of the radiometer.
According to the invention, switching sequences are selected, which are capable to select sequentially the array elements so that the output signals from the signal receivers can provide the four correlations to allow fully polarimetric operation of the radiometer. The switching sequences consist in at least four steps. A first switching sequence is assigned to a first signal receiver and a different switching sequence is assigned to each other signal receiver which defines a non-redundant baseline with said first signal receiver. Each remaining signal receiver is assigned a switching sequence such that each signal receiver of a distinct pair of signal receivers which define a non-redundant baseline is assigned a different switching sequence and such that each signal receiver of at least one distinct pair of signal receivers which define a redundant baseline is assigned a different switching sequence. Any of the selected switching sequences is assigned to the remaining signal receivers. Switch control signals indicating the switching sequences thus assigned are generated and applied to switch means operative to connect the signal receivers to the polarization ports of the array elements following the assigned switching sequences.
The polarization switching scheme of this invention is the shortest possible and thus achieves the minimum degradation in sensitivity as compare to dual channel interferometric polarimetric radiometers. The switching scheme is implemented in a switch control apparatus operative to control the switching operation in accordance with the selected switching sequences.
Another object of the present invention is to provide a single-channel aperture synthesis radiometer operating in a fully interferometric mode using the polarization switching scheme as outlined herein.
These and other objects and advantages of the present invention will become apparent from the following description and the accompanying drawings.